He moved to Poland, where he married the daughter of a leading member of the Polish Brethren, the anti-trinitarian minority, or ecclesia minor. In 1565 it had split from the Calvinist Reformed Church in Poland. Sozzini never joined the ecclesia minor, but was influential in reconciling several controversies among the Brethren: on conscientious objection, on prayer to Christ, and on the virgin birth. Fausto persuaded many in the Polish Brethren who were formerly Arian, such as Marcin Czechowic, to adopt his uncle Lelio's views.

Fausto Sozzini furthered his influence through his Racovian Catechism, published posthumously, which set out his uncle Lelio's views on Christology and replaced earlier catechisms of the Ecclesia Minor. His influence continued after his death through the writings of his students published in Polish and Latin from the press of the Racovian Academy at Raków, Kielce County.

In Britain and North America, "Socinianism" later became a catch-all term for any kind of Dissenting belief. Sources in the 18th and 19th centuries frequently attributed the term "Socinian" anachronistically, using it to refer to ideas that embraced a much wider range than the narrowly defined position of the Racovian catechisms and library.

The Socinians held that humans were created mortal in the beginning and would have died naturally whether Adam and Eve had eaten from the tree or not.[4] They also rejected the doctrine of original sin.[5]

The Socinians believed that God's omniscience was limited to what was a necessary truth in the future (what would definitely happen), and did not apply to what was a contingent truth (what might happen). They believed that, if God knew every possible future, human free will was impossible, and as such rejected the "hard" view of omniscience.[7]

Although not directly a doctrinal belief, the principle of conscientious objection and the obedient relation of the believer to the state became a distinct position of "Socinianism" as it was formalized in the Racovian publications. Before F. Sozzini's arrival in Poland there had been a wide range of positions from the total otherworldiness, common property, and withdrawal from the state of Marcin Czechowic of Lublin through to the advocacy of military service by Symon Budny. The next generation of Polish Brethren stabilized between these two positions, carrying wooden swords to follow the letter of the law, and allowing senior Socinians such as Hieronim Moskorzowski to vote in the Sejm.[12]

The direct doctrinal descendants of the original Socinians are the Unitarian Christians of Transylvania and England. Although the Polish Brethren never adopted the name "Unitarian" while in Poland, when they were disbanded in 1658, those who fled to Holland eventually embraced the term "Unitarian" (which they got from the Transylvanians), as they preferred not to be called "Socinians."[13] The term had been used by the Unitarian Church of Transylvania as early as 1600. Socinian theology continued in Transylvania, where Polish exiles such as Andrzej Wiszowaty Jr. taught in the Unitarian College (1726–1740), as evidenced in the Summa Universae Theologiae Christianae secundum Unitarios of Mihály Lombard de Szentábrahám recognized as the statement of faith of the Unitarian Church of Transylvania by Emperor Joseph II in 1782. Early English Unitarians such as Henry Hedworth and John Biddle retroactively applied the term "Unitarian" to the Polish Brethren. By 1676 there were at least three Socinian meeting houses in London, even if the Act of Toleration of 1689 saw Socinians and Catholics excluded from official recognition. Socinian ideas continued to have significant influence on Unitarians in England throughout the entire period of their development.

^Wulfert De Greef, The writings of John Calvin: an introductory guide, 2008. Quote: "Lelio Sozzini's Brevis explicatio in primum Johannis caput appeared in 1561, which marked the beginning of the Socinian phase among the Italian."

^"[Man] was originally created mortal; that is, was so constituted that he was not only by nature capable of dying, but also, if left to himself, could not but die." The Racovian Catechism (English trans. Thomas Rees, London 1818), p. 20.

^"The fall of Adam, as it was but one act, could not have power to deprave his own nature, much less that of his posterity." The Racovian Catechism (English trans. Thomas Rees, London 1818), p. 326.

^"Did not Christ die also, in order, properly speaking, to purchase our salvation, and literally to pay the debt of our sins? Although Christians at this time commonly so believe, yet this notion is false, erroneous, and exceedingly pernicious; since they conceive that Christ suffered an equivalent punishment for our sins, and by the price of his obedience exactly compensated our disobedience." The Racovian Catechism (English trans. Thomas Rees, London 1818), pp. 303–304.

^G. C. Berkouwer – 1954 The Person of Christ – Page 22 "The full consequences of this criticism, it is true, were not accepted in all respects, for the Socinians still believed that Christ was supernaturally begotten by way of the virgin birth, but the results of their criticism would soon

^Coleridge, philosophy and religion: Aids to reflection and the ... – Page 232 Douglas Hedley – 2000 "Socinian theology was severely critical of traditional dogmas. Faustus Socinus accepted the Virgin Birth, physical resurrection of Christ, and the exaltation to the right hand of God, but the orthodox Christological doctrine of two natures is rejected as contrary to sound reason (ratio sana)."

^Religion, secularization, and political thought: Thomas Hobbes to ... – Page 45 James E. Crimmins, Huron College – 1989 "Socinianism denied the Trinity (nowhere mentioned in the Bible), and with it the pre-existence of Christ before the virgin birth. It denied original sin and predestination, for those doctrines seemed to deny moral responsibility and ... "

^better source needed : The faiths of the world James Gardner – 1858 "Budny – He and his followers were not contented, like other Socinians, with denying the divinity of the Lord Jesus Christ, and affirming him to be a mere man, but they denied the inspiration of the Sacred Scriptures."

^Wilson Sects and Society Page 223 "7 The distinction of Christadelphian teaching from a unitarian position is apparent, although it shares much common ground with a Socinian or Arian position, yet with some differences. Christadelphians do not deny the divinity of Jesus, ... "

^Harry A. Lewis Peter Geach, philosophical encounters Peter Thomas Geach, 1991 "were and are widely believed in the Midlands; since in his time Socinians were liable to be burned, ... but Socinianism lives on under the new label of Christadelphianism, which has its main ecclesia in Birmingham. "

^The virtues – Page 62 Peter Thomas Geach – 1977 "The Socinians, or their modern successors like the Christadelphians, at least retain the traditional object of hope; the doctrine of going at death to another world in a spiritual body is an incoherent philosophical fantasy"

^Religious studies: Volume 17 Cambridge University Press. Online Journals – 1981 "Our Socinian contemporaries, the Christadelphians, are singularly lacking in what the eighteenth century censured as enthusiasm; to a serious enquirer they will argue about their beliefs with endless patience, courtesy, and ingenuity, ..."